amd64: handle small memset buffers with overlapping stores

amd64: handle small memset buffers with overlapping stores

Instead of jumping to locations which store the exact number of bytes,
use displacement to move the destination.

In particular the following clears an area between 8-16 (inclusive)
branch-free:

movq    %r10,(%rdi)
movq    %r10,-8(%rdi,%rcx)

For instance for rcx of 10 the second line is rdi + 10 - 8 = rdi + 2.
Writing 8 bytes starting at that offset overlaps with 6 bytes written
previously and writes 2 new, giving 10 in total.

Provides a nice win for smaller stores. Other ones are erratic depending
on the microarchitecture.

General idea taken from NetBSD (restricted use of the trick) and bionic
string functions (use for various ranges like in this patch).

Reviewed by:	kib (previous version)
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D17660